(521bh) Manipulating Inner and Outer Sphere Environments in Zeolites to Control Regioselectivity of Epoxide Methanolysis

The ring-opening of epoxides with alcohols produces high-value intermediates. Lewis and Brønsted acidic materials, including zeolites, catalyze this chemistry. However, few explanations of what factors govern differences in mechanisms, turnover rates, and regioselectivities between these materials exist. The effects of metal identity and solvation effects within zeolite pores must be considered for the design of better ring-opening catalysts. Here, we examine the consequences of varying the active metal (Al, Sn, Ti, Zr) on rates and regioselectivities for 1,2-epoxybutane (C₄H₈O) ring-opening with methanol (CH₃OH) in acetonitrile solvent (CH₃CN) over zeolite *BEA.

Among Lewis acids, ring-opening rates proceed in the order Ti < Zr < Sn, correlating with reported metal Lewis acid strengths. Rates depend on [CH₃OH] (where [x] denotes concentration) and [C₄H₈O]. A single kinetic regime appears to dominate at all conditions over Al-BEA-20, whereas each Lewis acid shows two kinetic regimes. These results suggest that Lewis and Brønsted acidic materials catalyze ring-opening with distinct prevailing surface species or through different mechanisms.

Regioselectivities towards the terminal ether product exceed 50% under most conditions but depend on both metal identity and reagent concentrations. Selectivity towards the terminal ether generally decreases with increasing [CH₃OH] and increases with greater [C₄H₈O] over each M-BEA-20. Al-BEA-20 consistently gives the lowest regioselectivities to the terminal ether, with Ti- and Zr-BEA-20 showing the greatest regioselectivities. Activation enthalpy and reactant and product adsorption enthalpies, obtained by isothermal titration calorimetry, suggest regioselectivities stem from differences in the free energies of the transition states to form each product. Methanol uptake measurements further suggest that differences in solvation between transition states contribute to regioselectivity changes.

This work demonstrates that regioselectivities for epoxide ring-opening are controlled by both inner sphere (i.e., choice of active metal, acid type) and outer sphere (i.e., intrapore solvent environment) interactions in zeolite pores.